In recent years the art in the field of telephone systems has been advanced through the use of tone switching systems in which the line switching and signalling information are transmitted through the telephone lines in the form of audio tones. In general, two pure tones are used in most switching systems to improve reliability and to prevent voice generated audio signals from confusing the tone detectors. In a key system, these tones originate from the depression of keys on a telephone keyboard.
An object of the present invention is to provide an improved signalling device for key telephone systems using tone detection which combines all circuit and logical functions in a single device. Another object of this invention is to provide an intercom signalling device which allows the connection of up to 19 intercom stations. Still another object of the invention is to provide an intercom signalling device which may utilize either single digit or two digit codes. A further object of the invention is to provide conference calling on both single digit and two digit systems utilizing the same device. Yet another object of the invention is to provide a device which permits conference calls to be established while the system is in use. Still another object of the invention is to provide a device in which the calling signals can be repeated without the necessity of operating the switch hook on the telephone instrument. Yet another object of the invention is to provide a device which includes a one second signal contact closure whenever an intercom signal is detected. Still another object of the invention is to provide a lock-out of incoming signals immediately following the detection of a calling tone. Yet another object of the invention is to provide a device which may be used with rotary dial equipment on the same line as the touch-tone system. Still another object of the invention is to provide a device which is so compact and self-contained that it may be installed in a fraction of the time and at less cost than prior art systems.
A further object of the invention is to provide noise, or "talk-down," protection in a system as above described using low group and high group touch tone signals, so that if a signal of any type is present in the slot between the low group and high group touch tone signals, the input to the signal decoders is shunted out and the time-out circuit of the system is disabled.
A still further object of the invention is to provide substantial reduction in the recognition time of at least the lowest frequency decoder of the system.